Move enterprise features under Channels (#19010)

* Move enterprise features under Channels

We move the EE features which are Channels related.

While here, we also move some code under *Server.Start()
from NewServer.

```release-note
NONE
```

* move saml and ldap back to server

```release-note
NONE
```

* fix test

```release-note
NONE
```

* try again

```release-note
NONE
```

app/server_test.go

@@ -182,11 +182,15 @@ func TestStartServerNoS3Bucket(t *testing.T) {
 				AmazonS3PathPrefix:      model.NewString(""),
 				AmazonS3SSL:             model.NewBool(false),
 			}
+			*cfg.ServiceSettings.ListenAddress = ":0"
 		})
 		return nil
 	})
 	require.NoError(t, err)
 
+	require.NoError(t, s.Start())
+	defer s.Shutdown()
+
 	// ensure that a new bucket was created
 	backend, appErr := s.FileBackend()
 	require.Nil(t, appErr)